Although we could use any valid signal-level loss function (see Equations REF , REF ), we choose the negative permutation-invariant scale-invariant signal to distortion ratio (SI-SDR) [1]}:
\({\begin{array}{c}\mathcal {L}(\widehat{y}, y) = - \text{SI-SDR}(\widehat{y}, y) = - 10 \log _{10} \left( \frac{\Vert  \alpha y\Vert ^2}{\Vert  \alpha y - \widehat{y}\Vert ^2} \right),\end{array}}\) 
